West Coast Expedition
July 20 - August 30, 2002
West Coast of North America
August 25, 2002: Day #37
Debra Stakes writes: After the disappointment of the previous dive, we decided to expand our search for potential seep sites to include two targeted dives in a single work day. Dives 473 and 474 were placed on the northern and southern sides of the Nootka Fracture Zone at its intersection with the Cascadia subduction zone. Each site would examine prospective sites of dewatering and or degassing of sediments as they are accreted to the subduction zone. We supposed that the transform intersection, by cleaving the accretionary slices, would induce zones of enhanced fluid flow. Dive 473 was placed along the northern wall of the Nootka "plunge pools" where the interior structure of the accreted slices would be exposed. The wall of the Nootka Fracture Zone proved to be many meters of tilted and somewhat deformed turbiditic sediments (Fig 1). Such sediments are characteristic of continental margins that serve as catchment basins for the eroding adjacent land. The movement into the subduction zone complex tilts the originally flat-laying sediments. Almost from the beginning of the dive, we observed dead clam shells mixed in with the eroding channels of siltstone and mudstone. But would we find the source of the shells today?
Near the summit of our vertical section, we observed an increasing abundance of cracks and fractures in the sedimentary rocks. Soon we were rewarded for our efforts with evidence of ongoing fluid flow in the form of groups of chemosynthetic tubeworms growing from the cracks (Fig. 2). At the top of the section, we finally discover groups of living clams nestled within one of the sedimentary layers. Both the sediments and the distribution of clams reminded me of the seeps in Monterey Bay (Fig 3). We mark the location with an acoustic beacon and head for the second area (Fig. 4)
Dive 474 covered much of the same sedimentary section as that observed on Dive 473. This dive transect covered the west-facing wall of the frontal accretionary wedge where the morphology suggested an extensive mass wasting event. Such events can expose deep stratigraphic sections with methane-rich pore fluids and permit these to vent laterally into the seafloor. We almost immediately found bunches of tubeworms attached to the broken sedimentary rocks (Fig. 5). Near the summit we found good evidence of methane seepage and oxidation - the cracks were filled with authigenic carbonate (Fig 6). We also found some local predators doing some seep faunal investigation on their own! (Fig. 6)